Imaging and automated image analysis
We have a large number of computer-controlled cameras which allow us to take images, record images sequences or make movies of foams and emulsions. These cameras are either attached to home-built set-ups which ensure the proper lighting and alignment conditions, or to other devices, such as our upright Olympus microscope. The microscope also allows us to do fluorescence imaging or phase contrast microscopy.
|Home-built imaging setup||Microscopy|
In order to treat the images we use automated image treatment, for which we tend to program our own modules using an image treatment software, such as ImageJ, as a basis. Such kind of image treatment allow us, for example, to follow how the size and distribution of bubbles or droplets changes with time.
High speed imaging
Certain processes of interest to us (such as bubble/droplet generation or coalescence) are so fast that we cannot analyse them using standard imaging techniques. For this purpose we therefore have two high-speed cameras in the lab which allow us to take thousands of images per second at a good resolution!
In order to characterise the ageing properties of foams, we have a commercial device, the FoamScan, from TECLIS in our lab. With the device we can generate foams in a controlled manner and follow the evolution of the foam height, its liquid fraction and the bubbles size in a fully automated manner. This technique allows us to scan and compare quickly the foaming properties of a large range of solutions.
In order to follow the evolution of the liquid fraction profile of a foam which undergoes drainage we have built a fully automatic device which measures the conductivity of a foam at up to 24 places along a foam column. A computer program relates the conductivity to the liquid fraction.
Static and dynamic light scattering
Foams are in general very turbid systems and it is difficult to ‘see’ inside them. However, using the principles of diffusing wave spectroscopy, the propagation of light through the foam can be used to measure the average bubble size in our foam and the dynamics of the foam. The average transmitted intensity is strongly dependent on the bubble size, while measuring the time-dependent fluctuations in the intensity of the light we can get information on the dynamics of rearrangements within the foam.
The Malvern Mastersizer is used to measure particle sizes in the range micrometric range. It works with the principle of laser diffraction: a laser beam is shone into a diluted sample and through measuring the angular intensity of the scattered light a particle size distribution can be obtained.
Foams and emulsions have complex mechanical properties. They can behave like an elastic solid or like a viscous fluid depending on the way they are deformed. To characerise their mechanical properties and those of the fluids which they are made of, we use standard bulk rheometers with cone-plate, plate-plate or couhaite geometries. We have three rheometers in the lab.
Acoustic properties of liquid and solid foams
The Brüel & Kjær Impedance tube allows us to measure the acoustic properties (absorption and sound velocity) of solid and liquid foams in a range of 50 – 6500 Hz in a fully automated manner.
We also have an automated home-built device to measure the velocity of sound in liquid in solid foams at 40 kHz. This sound velocity depends strongly on the density of the foam (Wood’s law) and can therefore be used to measure the liquid fraction of a foam.